From Rotating Stall to Surge: A Shock Tube Mechanism

Abstract

Compressor surge is a complete breakdown in compression resulting in an abrupt momentary reversal of gas flow and the violent pressure fluctuation with relatively low frequency and high amplitude. It commonly exists in dynamic type turbo compressors, particularly axial compressor and jet engine, or turbo charger for reciprocating engines. It is generally accepted that surge is preceded by a rotating stall, a situation of a few stalled blades rotating around compressor annulus (cascade) with much higher frequency. In jet engine, violent surge event typically produces a frightening loud bang, lots of vibrations and could cause catastrophic structural failures if not timely managed. Naturally, as important matters as rotating stall and surge, there have been tremendous R/D efforts from academia, government and industry devoted to this area, especially since jet engines became the prime powerhouses for modern airplanes. Despite of all the efforts, there still seems to be a more urgent need to understand the physical characteristics of the transition from a rotating stall to surge that has mystified researchers due to its transient nature. Fundamental questions remain unanswered even today, such as: What exactly triggers the surge to take place from a rotating stall? What is the physical nature of a compressor system or a local incipient surge: is it a movement of wave or fluid particles or both? How to estimate the quantitative destructive forces of a severe surge, that is, the maximum possible surge strength? This paper attempts to answer these questions by applying the classical Shock Tube Theory to the transient process from rotating stall to surge. The Shock Tube analogy is established with the hypothesis (implied from experimental observations) that an instant zero through flow condition exists inside a stalled cascade cell or dynamic compressor that triggers surge. It is revealed that surge event consists of a pair of non-linear compression and expansion waves (CW & EW) that instantly reverse gas flow (IRFF) by the pushing force of upstream propagating CW and the pulling force from downstream travelling EW. The surge strength is shown to be proportional to the square root of the pressure ratio of the involved cascade or compressor. Surge Rules are deduced to predict the location of surge initiation, the minimum and maximum surge strengths, travelling directions and speed. Moreover, a pro-active control strategy called SEWI (Surge Early Warning Initiative) is proposed using the unique characteristics of CW-IRFF-EW formation of a cascade cell induced surge as precursors for subsequent warning and controls before the destructive compressor surge takes place.